Knocking on Heaven's Door - Lisa Randall [176]
[ FIGURE 74 ] Pie chart illustrating the relative amounts of visible matter, dark matter, and dark energy of which the universe is composed.
Furthermore, gravitational influence and measurements indicate the presence of something even more mysterious than dark matter, known as dark energy. This is energy that permeates the universe, but doesn’t clump like ordinary matter or dilute as it expands. It is very much like the energy that precipitated inflation, but its density today is much smaller than it was back then.
Although we now live in a renaissance era of cosmology, in which theories and observations have advanced to the stage where ideas can be precisely tested, we also live in the dark ages. About 23 percent of the universe’s energy is carried by dark matter, and approximately another 73 percent is carried by the mysterious dark energy, as is illustrated in the pie chart. (See Figure 74.)
The last time something was called “dark” in physics was in the mid-1800s, when Urbain Jean Joseph Le Verrier of France proposed an unseen dark planet, which he named Vulcan. Leverrier’s goal was to explain the peculiar trajectory of the planet Mercury. Le Verrier, along with John C. Adams of England, had previously deduced the existence of Neptune based on its effects on the planet Uranus. Yet he was wrong about Mercury. It turned out that the reason for Mercury’s strange orbit was much more dramatic than the existence of another planet. The explanation could be found only with Einstein’s theory of relativity. The first confirmation that his theory of general relativity was correct was that he could use it to accurately predict Mercury’s orbit.
It could turn out that dark matter and dark energy are a consequence of known theories. But it might also be that these missing elements of the universe presage a similar significant change of paradigm. Only time will tell which of these options will resolve the dark matter and dark energy problems.
Even so, I’d say that dark matter is very likely to have a more conventional explanation, consistent with the type of physical laws we now know. After all, even if novel matter acts in accordance with force laws similar to those we know, why should all matter behave exactly like familiar matter? To put it more succinctly, why should all matter interact with light? If the history of science has taught us anything, it should be the shortsightedness of believing that what we see is all there is.
Many people think differently. They find dark matter’s existence very mysterious and ask how it can possibly be that most matter—about six times the amount we see—is something we can’t detect with conventional telescopes. Some are even suspicious that dark matter is really some sort of mistake. Personally, I think quite the opposite (though admittedly not even all physicists see it this way). It would perhaps be even more mysterious if the matter we can see with our eyes is all the matter that exists. Why should we have perfect senses that can directly perceive everything? Again, the lesson of physics over the centuries is how much is hidden from view. From this perspective, it’s mysterious why the stuff we do know should constitute even as much as 1/6 of the energy of all matter, an apparent coincidence that my colleagues and I are currently trying to understand.
We know something with dark matter’s properties has to be there. Although we don’t exactly “see” it, we do detect dark matter’s gravitational influence. We know dark matter exists due to the extensive observational evidence of its gravitational effects in the cosmos. The first clue that it existed came from the speed with which stars rotated in galaxy clusters. In 1933, Fritz Zwicky observed that galaxies in galaxy clusters orbited faster than